1. Field of the Invention
The present invention relates to an information recording medium using light, a method for recording information on the information recording medium, and an information recording device.
2. Description of Related Art
Various principles for recording information by irradiating light on a recording film have been known and among them, those using a change in atomic arrangement caused by heating, for example, phase change in a film material (also referred to as phase transition or phase transformation) have such an advantage that an information recording medium, on which information can be overwritten plural times, is achieved.
For example, as disclosed in JP-A No. 344807/2001, this type of phase-change optical disks are composed of a protective layer, a recording film made from a GeSbTe derivative, another protective layer, and reflective layer deposited on a substrate.
On the other hand, optical disks with a recording layer made from an organic material include CD-R and DVD-R. For this type of optical disks, information is recorded on them by irradiating a laser beam for transformation on the recording layer containing a dye, which may absorbs a wavelength of a recording light source, and the surface of the substrate in contact with the underside of the recording layer.
Another information recording method enabling ultra-high density recording by irradiating a variable wavelength laser, wherein a dye is coated around plural latex bulbs with slightly different sizes and resonance absorption takes place at individual wavelengths corresponding to their diameters, has been reported.
With a technique referred to as photo assist or thermal assist magnetic recording or magnetic-optical merged recording, information is recorded on the recording medium with an easy-to-magnetize axis perpendicular to its film surface at a high density by irradiating pulses or continuous laser beams while applying a modulated magnetic field from a coil and read out by a magnetic head compatible with the magnetic disk.
To record information on the optical disk, photo-assist magnetic disk, and thermal-assist magnetic disk at a high density, a binary recording or multi-level recording method, namely a modulation system where the lengths of recording marks finely adjusted, has been developed and enables high density recording in theory, though practically, it has such a problem that the start and end edges of the recoding mark is easy to shift due to various factors, disturbing the recording density from being improved.
Although it is preferable that the magnetic disk is multi-layered to improve the recording density effectively, it is difficult. Since in the case of a double-layered DVD, a focus point is changed in recording information and in the case of three or more layered DVD, laser power is lost by absorption and reflection, and spherical aberration is easy to develop.
For the multi-layer (3D) recording method using a 2-photon absorption effect of a transparent material on a recording light, required laser power is very large because a 2-photon absorption transition possibility is very low and a short pulse laser is required.
A wave length multiplexed recording method using latex bulbs has such problems that the distribution of the sizes of the bulbs cannot be controlled and a reproduced signal level increases depending on a wavelength. The object of the present invention is to solve these problems and achieve ultra-high density information recording.
Now, the configuration of the present invention to solve the above-mentioned problems is described below.
By building a periodic-structure with very low optical absorption and reflectance factors in the recording film of the recording medium or a layer adjacent the recording film using self-assembly adapted to the undulations on the substrate surface, the leading and trailing edges of the recording mark can be correctly positioned, achieving high density recording.
As the material for the self-assembly film, metal ultra-fine particles with, for example, thiol derivative organic molecules, oleyl amine molecules, and oleic acid molecules coated around them are used.
This type of material is self-assembled at film formation by depositing from a solution or by applying with. UV curing resign and heating to rearrange ultra-fine particles.
The arrays of the ultra-particles of the self-assembly film is preferably aligned in the direction of the tracking groove. This causes the inorganic substances of the ultra-fine particles and the organic substances around them to arrange alternately. To make the disk multi-layered, after the material is self-assembled, a thermal insulation spacer layer is deposited and another self-assembled film is formed.
The principle for recording information includes phase change or magnetization reversal or change in color by heating of the ultra-fine particles themselves as well as change in the mixed phase-change recording material or phase-change recording material with a film coated on it and change in color.
In the present invention, the concave part of the substrate is referred to as a grove. The portion between the grooves is referred to as a land.
When a light beam enters the film through the substrate, the grove looks to be convex-shaped from the viewpoint of an incident side.
For this reason, in the case of the system where the light beam enters the film from the opposite side of the substrate, a portion, which looks to be convex-shaped from the viewpoint of the incident side, may be also referred to as the groove.
However, this designation is opposite to the definition in the present invention because this portion looks to be convex-shaped when only the substrate is focused on, though in fact, it is the land defined between the grooves.
In the case of a so-called in-groove recording method where information is record on either the land or the groove, in many cases, better recording characteristics are exhibited when information is recorded on the convex portion from the viewpoint of the light incident side whether the light beam enters the film from the substrate side or from the opposite side of the substrate, through information may be recorded on the portion, which looks to be concave-shaped from the viewpoint of the light incident side, because no significant difference lies between two recording methods.
The multi-layer recording method of the present invention is also suitable for further improving a recording density.
Although it is preferable that the recording medium is made multi-layered to improve the effective recording density (effective surface density), in the case of conventional recording medium, either the quality of reproduced signals or recording sensitivity is bound to be sacrificed because spherical aberration tends to develop on the third layer or higher, as well as a tradeoff exist between transmittance and recording sensitivity of each layer.
Such a medium type that information is 3D-recorded including the direction of thickness on a transparent organic material has been known but for the type using 2-photon absorption, recording sensitivity is very low, while for the type using photopolymerization, storage-stability and recording sensitivity are low.
However, according to the preferred embodiment of the present invention, resonance absorption of the semiconductor laser beam can be induced to vary the wavelength of the beam sophisticatedly, for example, by accreting dye molecules around dielectric ultra-fine particles with different sizes for each layer, achieving optical absorption by the target layer only with no adverse influence of optical absorption at the light incident side.
The recording medium of the present invention is effective with recording density (track pitch, bit pitch) higher than that of the 2.6 GB DVD-RAM Standard and in particular, more effective with recording density higher than that of 4.7 GB DVD-RAM Standard.
If the wavelength of a light source is not around 660 nm (for example, it is around 400 nm or the numerical aperture (NA) of a condenser is not 0.65), it is effective with recording density higher than those converted into a wavelength ratio and NA ratio in both the radial and tangential directions.
Note that in this specification, the term xe2x80x9ca phase changexe2x80x9d is used to represent the phase changes, melting (change into the liquid phase)xe2x80x94re-crystallization and crystal-crystal state change in addition to crystal-amorphous.
The experiment, which gave an embodiment of the present invention, was conducted using a standard optical head receiving the light beam from the substrate side, though using a focusing lens with about 0.85 of NA and the head receiving the light beam from the opposite side of the substrate or a near-field head, high density recording suitable for the recording medium of the present invention can be achieved.